THIS invention relates to electronic radio frequency (RF) identification systems of the kind comprising a reader and a plurality of transponders. It more particularly relates to a protocol for reading the transponders in sequential manner.
It is well known that the total time required to read all the transponders in a transponder population is proportional to at least the number of transponders in the population and the length of the data stream (frame length) transmitted by each transponder in response to being prompted by the reader. Various schemes and/or protocols have been developed to improve the total read time. These include schemes to avoid collisions between data streams and protocols to switch off transponders already read.
In a first known protocol disclosed in U.S. Pat. No. 5,537,105, once a transponder has been read, it is acknowledged by the reader and switched to a sleep mode wherein it no longer transmits and hence no longer responds to the reader. In this way the number of still active transponders in the population is progressively decreased, thereby progressively reducing the probability of collisions.
A second known protocol is based on the first and in addition utilizes respective randomly generated hold-off periods before the transponders start responding with data frames to a prompt from the reader, thereby to reduce the probability of frame collisions, Furthermore, this protocol also utilizes a mute signal broadcast by the reader to mute all other transponders which would start to respond after a first or selected transponder onto which the reader has locked. However, due to the half duplex nature of these systems, it is not possible to mute those transponders which start responding after the selected transponder, but before the mute signal is received by them. All these non-muted transponders then continue to transmit their full frame lengths of typically 90–130 bits. This clearly results in collisions between the frames, a failure by the reader to read the selected or any other transponder and consequently there is no acknowledgment signal from the reader. The transponders only start retransmitting after a fixed time-out period. Thus, after a collision, the system recovery time is unnecessarily slow.
In a third known protocol, that disclosed in EU 689151 in the name of Kipp, the transponders process respective random hold-off time periods and then transmit a request-to-transmit (RTT) signal. If the reader responds with a first acknowledge signal in a particular time window following the RTT signal transmitted by a transponder, that transponder proceeds to transmit response data. Once this data has been read, the reader responds with a second acknowledge signal which may switch the transponder just read to a sleep mode. The problem with this protocol is that if more than one transponder start to transmit an RTT signal before the first acknowledge signal is received, the RTT signals are corrupted and the time lost is equal to at least the length of the RTT signal and a random hold-off time before a next transponder transmits an RTT signal. Hence, a system operating according to this protocol is vulnerable to corruption of an RTT signal originating from a transponder until the first acknowledge signal is received by the balance of the transponders.
In a fourth known protocol, that disclosed in U.S. Pat. No. 6,104,279 to Maletsky the transponders respond with a header first. The reader having received a header, broadcasts an acknowledge signal within a time window following the header successfully received. The transponder which transmitted the header interprets that acknowledge signal as a go-ahead signal and the other transponders are automatically and simultaneously muted. The problem with this protocol is that if more than one transponder start to transmit a header before the first acknowledge signal is received, the headers corrupt one another and the time lost is equal to at least the length of the header and a random hold-off time before a next transponder transmits a header. Hence, a system operating according to this protocol is vulnerable to corruption of a transmitted header until the acknowledge signal is received by the balance of the transponders.